1. Fields of the Invention
This invention relates to a signal transmission system, more particularly to a signal transmission system with signal transmission lines through each of which both a level signal and a superposed signal thereupon are transmitted between a central unit and each of a plurality of terminal units.
2. Description of the Prior Art
Conventional systems for transmitting signals between a plurality of terminal units and a central unit can be broadly classified into two types. One is a current level signal transmission system as has been employed in a conventional fire alarm system; the other is a time division multiplex system as has been employed in a known remote supervision and control system.
In the former type of signal transmission system, a plurality of fire detectors such as smoke detectors or heat detectors are connected to a fire alarm receiver through a common signal transmission line of two wires and connected in parallel with each other across the wires of said signal transmission line. When any one of the fire detectors detects the outbreak of a fire, it causes the signal transmission line to become shorted. The resulting increase in the line current is detected by the fire alarm receiver, where it is processed to produce an alarm indicative of fire presence, such as by issuing a warning sound. However, in such a conventional fire alarm system, it is impossible to detect the fire location, as the fire detector can only send a current level signal and is unable to send a signal showing the location of the fire detector.
The latter type of a signal transmission system, as is often called a TDM (Time Division Multiplex) system, has been widely employed in remote supervision and control systems. In this system, as disclosed in U.S. Pat. No. 4,139,737, a plurality of terminal units are connected to a central unit through a common signal transmission line of two wires and connected in parallel with each other across the wires of a signal transmission line. The central unit transmits address signals and control signals sequentially to respective ones of the terminal units. Each of the terminal units receives a corresponding address signal and a control signal transmitted from the central unit so as to be thereby controlled and transmits to the central unit a reply signal in accordance with the control signal. The address, control and reply signals are arranged in a series time band. In this system, each terminal unit is composed of a so-called intelligent type transmission device including a processor (CPU) to send and receive the transmission signal, however, such processor (CPU) is, as is well known, sometimes frail against high temperature caused by a fire. Accordingly, if such a signal transmission system is employed in a fire alarm system, the reliability of signal transmission will be reduced, as compared with the conventional fire alarm system. Therefore, it is most desirable to employ both types of signal transmission systems in combination.